Human facial structure evolved to tolerate punches to the head, according to new research that suggests our ancestors spent a lot of time fighting.

The study, published in the latest issue of the journal Biological Reviews, presents an alternative to the long-held theory that human faces look the way they do primarily because of a past evolved need among our ancestors to chew hard foods, like nuts.

Such ancestors likely included the australopiths, which lived 4 to 2 million years ago in Africa.

"The australopiths were characterized by a suite of traits that may have improved fighting ability, including hand proportions that allow formation of a fist; effectively turning the delicate musculoskeletal system of the hand into a club effective for striking," David Carrier, lead author of the study, said in a press release.

"If indeed the evolution of our hand proportions were associated with selection for fighting behavior you might expect the primary target, the face, to have undergone evolution to better protect it from injury when punched," added Carrier, who is a University of Utah biologist.

With that in mind, Carrier and colleague Michael Morgan, a University of Utah physician, studied both modern skulls and those of australopiths. They compared differences between males and females, and noted how facial bones respond to impacts.

The researchers found that bones that suffer the highest rates of fractures in fights are the same parts of the skull that exhibited the greatest increase in sturdiness during the evolution of our early human relatives. These bones are also the parts of the skull that show the greatest difference between males and females in both australopiths and humans today.

"In other words," Carrier said, "male and female faces are different because the parts of the skull that break in fights are bigger in males. Importantly, these facial features appear in the fossil record at approximately the same time that our ancestors evolved hand proportions that allow the formation of a fist."

He continued, "Together, these observations suggest that many of the facial features that characterize early hominins may have evolved to protect the face from injury during fighting with fists."

What were our prehistoric ancestors fighting about? Based on human behavior today and other primate behavior, it's likely that they often fought over mates, territory and other resources. They also might have just gotten on each other's nerves. After all, some were often cooped up for periods of time in caves and rock shelters.

If the latest theory holds true, then other ideas about human evolution go out the window. For example, French philosopher Rousseau argued that, before civilization, humans were noble savages and that civilization corrupted us, making us more violent.

Our distant past probably wasn't very tranquil, though.

"The hypothesis that our early ancestors were aggressive could be falsified if we found that the anatomical characters that distinguish us from other primates did not improve fighting ability," Carrier said. "What our research has been showing is that many of the anatomical characters of great apes and our ancestors, the early hominins (such as bipedal posture, the proportions of our hands and the shape of our faces) do, in fact, improve fighting performance."

The researchers hope their findings could one day help to resolve problems associated with human aggression, inborn or not.

"Our research is about peace," Morgan explained. "We seek to explore, understand, and confront humankind's violent and aggressive tendencies."

"Peace begins with ourselves and is ultimately achieved through disciplined self-analysis and an understanding of where we've come from as a species. Through our research we hope to look ourselves in the mirror and begin the difficult work of changing ourselves for the better."

To put a human face on our ancestors, scientists from the Senckenberg Research Institute used sophisticated methods to form 27 model heads based on tiny bone fragments, teeth and skulls collected from across the globe.
The heads are on display for the first time together at the Senckenberg Natural History Museum in Frankfurt, Germany.
This model is Sahelanthropus tchadensis, also nicknamed "Toumai," who lived 6.8 million years ago. Parts of its jaw bone and teeth were found nine years ago in the Djurab desert in Chad. It's one of the oldest hominid specimens ever found.

With each new discovery, paleoanthropologists have to rewrite the origins of man's ancestors, adding on new branches and tracking when species split.
This model was fashioned from pieces of a skull and jaw found among the remains of 17 pre-humans (nine adults, three adolescents and five children) which were discovered in the Afar Region of Ethiopia in 1975.
The ape-man species, Australopithecus afarensis, is believed to have lived 3.2 million years ago. Several more bones from this species have been found in Ethiopia, including the famed "Lucy," a nearly complete A. afarensis skeleton found in Hadar.

Meet "Mrs. Ples," the popular nickname for the most complete skull of an Australopithecus africanus, unearthed in Sterkfontein, South Africa in 1947.
It is believed she lived 2.5 million years ago (although the sex of the fossil is not entirely certain).
Crystals found on her skull suggest that she died after falling into a chalk pit, which was later filled with sediment.
A. africanus has long puzzled scientists because of its massive jaws and teeth, but they now believe the species' skull design was optimal for cracking nuts and seeds.

The skull of this male adult was found on the western shore of Lake Turkana in Kenya in 1985. The shape of the mouth indicates that he had a strong bite and could chew plants.
He is believed to have lived in 2.5 million years ago and is classified as Paranthropus aethiopicus. Much is still unknown about this species because so few reamins of P. aethiopicus have been found.

Researchers shaped this skull of "Zinj," found in 1959. The adult male lived 1.8 million years ago in the Olduvai Gorge of Tanzania.
His scientific name is Paranthropus boisei, though he was originally called Zinjanthropus boisei -- hence the nickname.
First discovered by anthropologist Mary Leakey, the well-preserved cranium has a small brain cavity.
He would have eaten seeds, plants and roots which he probably dug with sticks or bones.

This model of a sub-human species -- Homo rudolfensis -- was made from bone fragments found in Koobi Fora, Kenya, in 1972.
The adult male is believed to have lived about 1.8 million years ago. He used stone tools and ate meat and plants.
H. Rudolfensis' distinctive features include a flatter, broader face and broader postcanine teeth, with more complex crowns and roots. He is also recognized as having a larger cranium than his contemporaries.

The almost perfectly preserved skeleton of the "Turkana Boy" is one of the most spectacular discoveries in paleoanthropology. Judging from his anatomy, scientists believe this Homo ergaster was a tall youth about 13 to 15 years old.
According to research, the boy died beside a shallow river delta, where he was covered by alluvial sediments.
Comparing the shape of the skull and teeth, H. ergaster had a similiar head structure to the Asian Homo erectus.

This adult male, Homo heidelbergensis, was discovered in in Sima de los Huesos, Spain in 1993. Judging by the skull and cranium, scientists believe he probably died from a massive infection that caused a facial deformation.
The model, shown here, does not include the deformity.
This species is believed to be an ancestor of Neanderthals, as seen in the shape of his face.
"Miquelon," the nickname of "Atapuerca 5", lived about 500,000 to 350,000 years ago and fossils of this species have been found in Italy, France and Greece.

The "Old Man of La Chapelle" was recreated from the skull and jaw of a Homo neanderthalensis male found near La Chapelle-aux-Saints, in France in 1908. He lived 56,000 years ago.
His relatively old age, thought to be between 40 to 50 years old, indicates he was well looked after by a clan.
The old man's skeleton indicates he suffered from a number of afflictions, including arthritis, and had numerous broken bones.
Scientists at first did not realize the age and afflicted state of this specimen when he was first discovered. This led them to incorrectly theorize that male Neanderthals were hunched over when they walked.

The skull and jaw of this female "hobbit" was found in Liang Bua, Flores, Indonesia, in 2003. She was about 1 meter tall (about 3'3") and lived about 18,000 years ago.
The discovery of her species, Homo floresiensis, brought into question the belief that Homo sapiens was the only form of mankind for the past 30,000 years.
Scientists are still debating whether Homo floresiensis was its own species, or merely a group of diseased modern humans. Evidence is mounting that these small beings were, in fact, a distinct human species.

Bones can only tell us so much. Experts often assume or make educated guesses to fill in the gaps in mankind's family tree, and to develop a sense what our ancestors may have looked like.
Judging from skull and mandible fragments found in a cave in Israel in 1969, this young female Homo sapien lived between 100,000 and 90,000 years ago. Her bones indicate she was about 20 years old. Her shattered skull was found among the remains of 20 others in a shallow grave.